1. Technical Field
The present invention relates to a polymer electrolyte fuel cell, and in particular, relates to a technique for improving power generation efficiency in high-voltage operation.
2. Background Art
Recently, exhaustion of petroleum resources is a crucial issue, and environmental problems such as air pollution and global warming caused by consumption of fossil fuels have become serious. Under these circumstances, fuel cells have attracted much attention as a clean power source for electric motors in which carbon dioxide is not generated, and such fuel cells are being widely developed and used.
In the case in which such a fuel cell is used in a vehicle, a polymer electrolyte fuel cell in which a polymer electrolyte membrane is used is desirably used since high voltage and large current can be obtained. A membrane electrode assembly for the polymer electrolyte fuel cell is produced as follows: a catalyst such as platinum is carried by a catalyst carrier such as carbon black; a pair of electrode catalytic layers is made by unifying the catalyst and an ion conducting polymer binder; a polymer electrolyte membrane having ion conductivity is disposed between the electrode catalytic layers; and a gas-diffusion layer is formed on each of the electrode catalytic layers. Furthermore, a separator which also functions as a gas passage is formed on each of the gas-diffusion layers to yield a polymer electrolyte fuel cell.
In such a polymer electrolyte fuel cell, a reducing gas, such as hydrogen or methanol, is introduced at one electrode catalytic layer (fuel electrode) through the gas-diffusion layer of the fuel electrode side, and an oxidizing gas such as air or oxygen is introduced at the other electrode catalytic layer (oxygen electrode) through the gas-diffusion layer of the oxygen electrode side. In the fuel electrode, due to the existence of the catalyst in the electrode catalytic layer, protons (H+) and electrons are generated from the reducing gas, and protons migrate to the electrode catalytic layer of the oxygen electrode side through the polymer electrolyte membrane. In the oxygen electrode, due to the existence of the catalyst in the oxygen electrode, protons react with the oxidizing gas introduced at the oxygen electrode and electrons to produce water. Therefore, by electrically connecting the fuel electrode and the oxygen electrode with a lead, a circuit in which electrons generated in the fuel electrode migrate to the oxygen electrode is formed, and electric current is obtained.
At the cathode of the polymer electrolyte fuel cell, water is generated by reduction reaction of protons and oxygen which is an oxidizer. Therefore, in a high-voltage operation, water is generated faster than water vapor at the cathode can be exhausted, and as a result, a blockage (flooding) phenomenon of micropores of the electrode by condensation of water vapor may easily occur. If the flooding phenomenon occurs, the number of reaction sites is reduced, the reaction cannot be promoted efficiently, and it would be difficult to obtain reliable properties for a long period.
To solve such a problem, a technique in which graphitized carbon supporting a noble metal which is a catalyst component of the cathode is used, is disclosed (see Japanese Unexamined Patent Application Publication No. 2000-268828, 2000-357857, 2002-015745). In the technique, since the graphite is water-repellent, water is exhausted immediately, and the flooding can be restrained.
However, the number of functional groups on the surface of the supporting body and specific surface area are reduced by a heat treatment of graphitization, and as a result, the water holding property of graphite in the electrode is deteriorated. Therefore, under a low humidity operation, i.e., not more than 50% humidity, required water is in short supply, an activation over voltage of catalytic reaction is increased, and power generation efficiency is deteriorated.